Basecoat and associated paperboard structure

ABSTRACT

A basecoat including a pigment blend that includes a ground calcium carbonate component and a hyperplaty clay component, wherein the ground calcium carbonate component has a coarse particle size distribution, and wherein the hyperplaty clay component has an average aspect ratio of at least 40:1.

PRIORITY

This application is a continuation of U.S. Ser. No. 13/404,151(pending), which is a continuation of U.S. Ser. No. 12/326,430 filed onDec. 2, 2008, now U.S. Pat. No. 8,142,887 which claims priority fromU.S. Ser. No. 61/038,579 (expired) filed on Mar. 21, 2008. The entirecontents of all three priority applications are incorporated herein byreference.

FIELD

The present patent application is directed to coatings for paperboardand, more particularly, to basecoats as well as smooth paperboardstructures formed with the disclosed basecoats.

BACKGROUND

Paperboard is used in various packaging applications. For example,aseptic liquid packaging paperboard is used for packaging beveragecartons, boxes and the like. Therefore, customers often preferpaperboard having a generally smooth surface with few imperfections tofacilitate the printing of high quality text and graphics, therebyincreasing the visual appeal of products packaged in paperboard.

Conventionally, paperboard smoothness is achieved by a wet stackcalendering process in which the paperboard is rewetted and passedthrough a calendering device having two or more hard rolls. The wetstack calendering process smoothes the paperboard by compressing thefiber network to reduce the pits and crevices in raw stock paperboard(see FIG. 1). The result is a smooth paperboard with reduced boardthickness and bulk and, therefore, reduced stiffness. However, stiffnessis an important requirement for many paperboard applications, such asaseptic liquid packaging paperboard. Therefore, preparing a smooth yetstiff paperboard using the conventional wet stack calendering processrequires increasing the basis weight of the paperboard, therebysubstantially increasing the raw material cost.

Alternatively, manufacturers have attempted to smooth the surface ofpaperboard by coating the entire surface of the paperboard with abasecoat comprised of various pigments, such as clay, calcium carbonate,TiO.sub.2 and the like, then overcoating this base with a second andsometimes even a third coating, which is generally referred to as atopcoat. It was discovered that high quantities of relatively finepigment particles applied to the surface of paperboard provided a moresmooth surface without the need for wet stack calendering, therebymaintaining bulk. For example, as shown in FIG. 2, it was discoveredthat relatively high quantities (e.g., 10.6 pounds per 3000 ft.sup.2 ormore) of relatively fine ground calcium carbonate, such as CARBITAL®95(Imerys Pigments, Inc. of Roswell, Ga.), applied to the rough surface ofpaperboard provided the greatest smoothness. Indeed, it has beenunderstood that the more pigment applied to the surface of thepaperboard the better the resulting smoothness. However, the use ofrelatively high quantities of pigments substantially increases the costof preparing smooth and highly printable paperboard.

Accordingly, there is a need for a basecoat and associated paperboardstructure that maintains paperboard bulk and provides the desiredsmoothness for high quality printing, while reducing manufacturing cost.

SUMMARY

In one aspect, the disclosed basecoat may include a pigment blend of acoarse ground calcium carbonate and a hyperplaty clay having an averageaspect ratio of at least about 40:1.

In another aspect, a paperboard substrate may be coated with thedisclosed basecoat to form a coated paperboard structure.

Other aspects of the disclosed basecoat and associated paperboardstructure will become apparent from the following description, theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of an uncoated surface of an exemplary paperboardsubstrate (i.e., raw stock);

FIG. 2 is a photographic comparison of the surface of a paperboardsubstrate coated with various quantities (in pounds per 3000 ft.sup.2)of fine ground calcium carbonate according to the prior art;

FIG. 3 is a photographic comparison of the surface of a paperboardsubstrate coated with various quantities (in pounds per 3000 ft.sup.2)of the disclosed basecoat;

FIG. 4 is a graphical illustration of percent sediment void volumeversus percent clay component for various pigment blends formulated withan extra course ground calcium carbonate;

FIG. 5 is a graphical illustration of percent sediment void volumeversus percent clay component for various pigment blends formulated witha course ground calcium carbonate;

FIG. 6 is a graphical illustration of percent sediment void volumeversus percent clay component for various pigment blends formulated witha fine ground calcium carbonate;

FIG. 7 is a first graphical comparison of Parker Print Surfacesmoothness versus coat weight;

FIG. 8 is a second graphical comparison of Parker Print Surfacesmoothness versus coat weight;

FIG. 9 is a side cross-sectional view of a paperboard substrate coatedwith the disclosed basecoat according to the disclosed method; and

FIG. 10 is a side cross-sectional view of the paperboard substrate ofFIG. 9 shown at a second, greater magnification.

DETAILED DESCRIPTION

In one aspect, the disclosed basecoat may include a pigment blend ofhigh aspect ratio clay and calcium carbonate. The pigment blend may bedispersed in a carrier, such as water, to facilitate application of thebasecoat to an appropriate substrate, such as a paperboard substrate.Additional components, such as binders, stabilizers, dispersing agentsand additional pigments, may be combined with the pigment blend to formthe final basecoat without departing from the scope of the presentdisclosure.

As used herein, “paperboard substrate” broadly refers to any paperboardmaterial that is capable of being coated with the disclosed basecoat.Those skilled in the art will appreciate that the paperboard substratemay be bleached or unbleached, and typically is thicker and more rigidthan paper. Generally, a paperboard substrate has an uncoated basisweight of about 85 pounds per 3000 ft.sup.2 or more. Examples ofappropriate paperboard substrates include corrugating medium, linerboardand solid bleached sulfate (SBS).

The clay component of the pigment blend of the disclosed basecoat may beany platy clay having a relatively high aspect ratio or shape factor(i.e., hyperplaty clay). As used herein, the terms “aspect ratio” and“shape factor” refer to the geometry of the individual clay particles,specifically to a comparison of a first dimension of a clay particle(e.g., the diameter or length of the clay particle) to a seconddimension of the clay particle (e.g., the thickness or width of the clayparticle). The terms “hyperplaty,” “high aspect ratio” and “relativelyhigh aspect ratio” refer to aspect ratios generally in excess of 40:1,such as 50:1 or more, particularly 70:1 or more, and preferably 90:1 ormore.

In one aspect, the clay component of the pigment blend may include aplaty clay wherein, on average, the clay particles have an aspect ratioof about 40:1 or more. In another aspect, the clay component may includea platy clay wherein, on average, the clay particles have an aspectratio of about 50:1 or more. An example of such a clay is CONTOUR® 1180available from Imerys Pigments, Inc. of Roswell, Ga. In another aspect,the clay component may include a platy clay wherein, on average, theclay particles have an aspect ratio of about 90:1 or more. An example ofsuch a clay is XP-6100 also available from Imerys Pigments, Inc.Additional examples of appropriate platy clays are disclosed in U.S.Pat. No. 7,208,039 to Jones et al., the entire contents of which areincorporated herein by reference.

In another aspect, the clay component of the pigment blend may includeplaty clay having a relatively high average particle size. In oneparticular aspect, the clay component may have an average particle sizeof about 4 microns or more. In a second particular aspect, the claycomponent may have an average particle size of about 10 microns or more.In a third particular aspect, the clay component may have an averageparticle size of about 13 microns or more.

The calcium carbonate component of the pigment blend of the disclosedbasecoat may include a ground calcium carbonate. In one aspect, thecalcium carbonate component may include a fine ground calcium carbonate.An example of such a fine ground calcium carbonate is CARBITAL® 95,available from Imerys Pigments, Inc. of Roswell, Ga., wherein about 95percent of the calcium carbonate particles are less than about 2 micronsin diameter. In another aspect, the calcium carbonate component mayinclude a coarse ground calcium carbonate. An example of such a coarseground calcium carbonate is CARBITAL® 60, also available from ImerysPigments, Inc., wherein about 60 percent of the calcium carbonateparticles are less than about 2 microns in diameter. In another aspect,the calcium carbonate component may include an extra coarse groundcalcium carbonate. An example of such an extra coarse ground calciumcarbonate is CARBITAL® 35, also available from Imerys Pigments, Inc.,wherein only about 35 percent of the calcium carbonate particles areless than about 2 microns in diameter.

In another aspect, the calcium carbonate component of the pigment blendmay have an average particle size of about 1 micron or more, such asabout 1.5 microns and, more particularly, 3 microns or more.

Without being limited to any particular theory, it is believed thatpigment blends that are formulated to provide relatively high percentsediment void volumes (i.e., bulkier particle packing) provide highsmoothness at relatively low coat weights, thereby reducing raw materialcosts. Furthermore, it is believed that using a clay component having arelatively high aspect ratio and/or a relatively high average particlesize and a calcium carbonate component having a relatively high averageparticle size yields relatively high and, therefore, desirable percentsediment void volumes. For example, sediment void volumes in excess of45 percent may be desired, while sediment void volumes in excess of 47.5percent may be more desired and sediment void volumes in excess of 50percent may be even more desired.

One appropriate technique for measuring percent sediment void volumeincludes preparing a pigment blend sample having the desired weightpercentage of the clay component to the calcium carbonate component. Thepigment blend sample is then diluted with water to 50 percent by weightsolids to provide a slurry. A 70 gram sample of the slurry is placedinto a centrifuge tube and spun at about 8000 g for about 90 minutes.The sample is then removed from the centrifuge and the clear supernatantliquid is separated and weighed. The sediment is typically packeddensely enough that the supernatant liquid is easy to pour off. Basedupon the weight of the water removed, the weight of water stillcontained in the voids of the sediment may be calculated. Then, usingparticle densities, the weight of water in the voids may be convertedinto percent sediment void volume.

Referring to FIGS. 4-6, the percent sediment void volume for variouspigment blends versus the percent by weight of the clay component in thepigment blend is provided. Specifically, FIGS. 4-6 compare the use ofCARBITAL® 35 (extra coarse), CARBITAL® 60 (coarse) and CARBITAL® 95(fine) as the calcium carbonate component and XP-6100 (aspect ratio over90:1), CONTOUR® 1180 (aspect ratio about 50:1), CONTOUR® Xtrm (aspectratio about 45:1) and KCS (aspect ratio about 10:1 (not a high aspectratio clay)) as the clay component.

FIGS. 4-6 indicate that coarse ground calcium carbonate (FIGS. 4 and 5),particularly extra coarse ground calcium carbonate (FIG. 4), and highaspect ratio clays, particularly clays having an aspect ratio over 70:1,more particularly over 90:1 (XP-6100 clay), provide the highest percentsediment void volume.

Furthermore, the concave shape of the curves in FIGS. 4-6, particularlythe curves associated with XP-6100 clay, indicates that maximum percentsediment void volume is achieved when the clay component is blended withthe calcium carbonate component. For example, referring to FIG. 4, whenextra coarse ground calcium carbonate and XP-6100 are used, maximumpercent sediment void volume occurs between about 60 and about 90percent by weight of the clay component.

Still furthermore, the concave shape of the curves indicates thatcertain blends of the clay component and the calcium carbonate componentprovide a percent sediment void volume that is similar, if not higher,than using 100 percent high aspect ratio clay. Therefore, the curvesindicate that blending less expensive calcium carbonate with moreexpensive high aspect ratio clay may yield an equal, if not superior,coating material in terms of percent sediment void volume. Indeed,comparing FIG. 4 to FIG. 6 for example, the curves indicate that thecoarser the calcium carbonate, the less high aspect ratio clay must beused to achieve higher percent sediment void volume. For example,referring to FIG. 4, when extra coarse ground calcium carbonate isblended with XP-6100 clay, a 45:55 blend of the clay component to thecalcium carbonate component provides the same percent sediment voidvolume as 100 percent of the high aspect ratio clay.

Referring to FIGS. 7 and 8, the Parker Print Surface (“PPS”) smoothnessvalues of paperboard coated with various basecoats on a pilot coater arepresented with respect to the coat weight of the basecoat in pounds perream (3000 ft.sup.2). Those skilled in the art will appreciate that PPSsmoothness values taken from samples prepared with a pilot coater aregenerally higher than the PPS smoothness values obtained from samplesprepared on a full scale mill. Nonetheless, the PPS smoothness valuestaken using a pilot coater are indicative of the improvement provided bythe disclosed basecoats over prior art coatings. For reference, when apilot coater is used, PPS smoothness values of about 7.0 microns or lessare generally desired, PPS smoothness values of about 6.5 microns orless are preferred and PPS smoothness values of about 6.0 microns orless are more preferred.

Of particular interest, as shown in FIG. 7, basecoats including coarseor extra course calcium carbonate and high aspect ratio clay,particularly XP-6100 clay, provide relatively high percent sediment voidvolumes and present PPS smoothness values generally below about 7microns at coat weights of about 9 pounds per ream or less on apaperboard substrate. Indeed, as shown by the positive slope of thecurves in FIG. 7, improved smoothness (i.e., lower PPS smoothness value)of the resulting paperboard is directly correlated to lower coatweights. This data is contrary to the expectations of those skilled inthe art, which would expect higher smoothness values at high coatweights.

Indeed, when a full scale mill was used, a basecoat including a 50:50pigment blend of CARBITAL® 35 (ground calcium carbonate) and XP-6100(high aspect ratio and high average particle size clay) yielded a PPSsmoothness value of about 2 microns at a relatively low coat weight of 6pounds per ream.

Accordingly, coating substrates such as paperboard with basecoatscomprising ground calcium carbonate, particularly coarse or extra courseground calcium carbonate, and high aspect ratio clay, particularly clayhaving an aspect ratio in excess of about 70:1, more particularly highaspect ratio clay having a relatively high average particle size, yieldsa smooth paperboard structure without sacrificing bulk, and reducesmanufacturing cost by combining more expensive platy clay with lessexpensive ground calcium carbonate, while requiring surprisingly lowcoat weights to achieve the desired smoothness.

Furthermore, those skilled in the art will appreciate that the type ofhigh aspect ratio clay selected and the type of ground calcium carbonateselected, as well as the ratio of the clay component to the calciumcarbonate component, may be dictated by cost considerations in view ofthe desired smoothness.

The disclosed basecoats may be applied to the surface of a substrate,such as paperboard (e.g., aseptic liquid packaging paperboard), in aquantity sufficient to fill the pits and crevices in the substratewithout the need for coating the entire surface of the substrate.Therefore, the disclosed basecoat together with the disclosed method forapplying the basecoat may be used to obtain high surface smoothness witha relatively small quantity of basecoat. Indeed, as discussed above,high surface smoothness may be achieved with an unexpectedly smallquantity of the disclosed basecoat.

In one aspect, the basecoat is applied to the substrate using a bladecoater such that the blade coater urges the basecoat into the pits andcrevices in the substrate while removing the basecoat from the surfaceof the substrate. Specifically, as shown in FIGS. 9 and 10, the basecoatmay be applied in a manner that is more akin to spackling, whereinsubstantially all of the basecoat resides in the pits and crevices inthe surface of the substrate rather than on the surface of thesubstrate.

At this point, those skilled in the art will appreciate that when thedisclosed basecoat is used in a blade coater the spacing between themoving substrate and the blade of the coater may be minimized tofacilitate filling the pits and crevices in the surface withoutsubstantially depositing the basecoat on the surface of the substrate(i.e., forming a discontinuous film on the surface of the substrate). Inother words, the blade of the coater may be positioned sufficientlyclose to the surface of the moving substrate such that the blade of thecoater urges the basecoat into the pits and crevices in the surface ofthe substrate, while removing excess basecoat from the surface of thesubstrate.

Example 1

A first pigment blend prepared according to an aspect of the presentdisclosure includes 50 percent by weight CARBITAL® 35 (coarse groundcalcium carbonate) and 50 percent by weight XP-6100 (hyperplaty clay).In a stationary mixer, a coating formulation is prepared by combiningthe 50:50 pigment blend with water, latex binders and a thickeningagent. The water is added in a quantity sufficient to form a slurry.Using a blade coater in the manner described above, the coatingformulation is applied to raw paperboard stock having a basis weight ofabout 126 pounds per 3000 ft.sup.2 at the following coat weights: 6.7,7.9, 8.9 and 11.3 pounds per 3000 ft.sup.2. Photographic results areshown in FIG. 3 and the PPS smoothness values are provided in FIG. 7(data points marked with a circle).

Thus, as shown in FIG. 3, the disclosed basecoat and associated methodprovide optimum smoothness at relatively low coat weights. (Compare FIG.2 to FIG. 3.) Specifically, the greatest smoothness is achieved at acoat weight of 6.7 pounds per 3000 ft.sup.2, with good smoothnessachieved at 7.9 pounds per 3000 ft.sup.2, with less smoothness at 8.9pounds per 3000 ft.sup.2, and even less smoothness at 11.3 pounds per3000 ft.sup.2.

Example 2

A second pigment blend prepared according to an aspect of the presentdisclosure includes 50 percent by weight OMYA HYDROCARB® 60 (coarseground calcium carbonate available from Omya AG of Oftringen,Switzerland) and 50 percent by weight XP-6170 (hyperplaty clay availablefrom Imerys Pigments, Inc.). In a stationary mixer, a coatingformulation is prepared by combining the 50:50 pigment blend with water,latex and starch binders and a thickening agent. The water is added in aquantity sufficient to form a slurry. Using a blade coater in the mannerdescribed above, the coating formulation is applied to raw paperboardstock having a basis weight of about 106 pounds per 3000 ft.sup.2 atcoat weights of 5.8 and 6.8 pounds per 3000 ft.sup.2, thereby providingpaperboard structures with improved smoothness at relatively low coatweights.

Accordingly, at this point those skilled in the art will appreciate thatbasecoats formulated according to the present disclosure to includecoarse ground calcium carbonate, particularly extra coarse groundcalcium carbonate, and hyperplaty clay, particularly hyperplaty clayshaving aspect ratios in excess of about 70:1, and more particularly highaspect ratio clays having a relatively high average particle size (e.g.,about 10 microns or more), provide increased surface smoothness atrelatively low coat weights, particularly when applied to the substrateusing the disclosed method.

While the pigment blends discussed above include platy clay and groundcalcium carbonate, particularly coarse ground calcium carbonate, thoseskilled in the art will appreciate that alternative pigment blends maybe used without departing from the scope of the present disclosure. Forexample, the pigment blend of the disclosed basecoat may include a platyclay and one or more additional inorganic pigments other than groundcalcium carbonate, such as precipitated calcium carbonate, talc orkaolin clay.

Although various aspects of the disclosed basecoat and associatedpaperboard structure have been shown and described, modifications mayoccur to those skilled in the art upon reading the specification. Thepresent patent application includes such modifications and is limitedonly by the scope of the claims.

1. A method of preparing a basecoat, the method comprising: providing apigment blend comprising a ground calcium carbonate component and aplaty clay component, wherein at most 60 percent of the ground calciumcarbonate component has a particle size smaller than 2 microns, andwherein the platy clay component has an average aspect ratio of at least40:1; providing a carrier; and dispersing the pigment blend in thecarrier.
 2. The method of claim 1, comprising the further step ofproviding at least one additional component and dispersing the at leastone additional component in the carrier.
 3. The method of claim 2,wherein the at least one additional component is selected from the groupconsisting of latex, starch, binders, stabilizers, dispersing agents,thickening agents, and additional pigments.
 4. The method of claim 1,wherein the carrier comprises water in a quantity sufficient to form aslurry.
 5. The method of claim 1 wherein the average aspect ratio of theplaty clay component is at least 50:1.
 6. The method of claim 1 whereinthe average aspect ratio of the platy clay component is at least 70:1.7. The method of claim 1 wherein the average aspect ratio of the platyclay component is at least 90:1.
 8. The method of claim 1 wherein atmost 35 percent of the ground calcium carbonate component has a particlesize smaller than 2 microns.
 9. The method of claim 1 wherein the groundcalcium carbonate component is at least 10 percent by weight of thepigment blend and at most 60 percent by weight of the pigment blend. 10.The method of claim 1 wherein the pigment blend consists essentially ofthe platy clay component and the ground calcium carbonate component. 11.The method of claim 1 wherein the pigment blend has a sediment voidvolume of at least 45 percent, wherein the sediment void volume ismeasured by preparing a pigment blend sample having the desired weightpercentage of the clay component to the calcium carbonate component;diluting the pigment blend sample with water to 50 percent by weightsolids to provide a slurry; placing a 70 gram sample of the slurry intoa centrifuge tube; spinning the sample at about 8000 g for about 90minutes; removing the sample from the centrifuge; separating the clearsupernatant liquid from the sample; weighing the clear supernatantliquid; determining the weight of water still contained in the voids ofthe sediment based on the weight of the clear supernatant liquidremoved; and using particle densities, converting the weight of water inthe voids into percent sediment void volume.
 12. The method of claim 11wherein the pigment blend has a sediment void volume of at least 47.5percent.
 13. The method of claim 12 wherein the pigment blend has asediment void volume of at least 50 percent.
 14. A method of preparing abasecoat, the method comprising: providing a pigment blend comprising aground calcium carbonate component and a platy clay component, whereinat most 60 percent of said ground calcium carbonate component has aparticle size smaller than 2 microns, wherein the ground calciumcarbonate component is at least 25 percent by weight of said pigmentblend and at most 60 percent of the pigment blend; wherein said platyclay component has an average aspect ratio of at least 40:1; whereinsaid pigment blend has a sediment void volume of at least 45 percent;providing a carrier; and dispersing the pigment blend in the carrier.15. The method of claim 14, comprising the further step of providing atleast one additional component and dispersing the at least oneadditional component in the carrier, wherein the at least one additionalcomponent is selected from the group consisting of latex, starch,binders, stabilizers, dispersing agents, thickening agents, andadditional pigments.
 16. The method of claim 14, wherein the carriercomprises water in a quantity sufficient to form a slurry.
 17. Themethod of claim 14, wherein the average aspect ratio of the platy claycomponent is at least 50:1.
 18. The method of claim 14, wherein at most35 percent of the ground calcium carbonate component has a particle sizesmaller than 2 microns.
 19. The method of claim 14, wherein the groundcalcium carbonate component is at least 10 percent by weight of thepigment blend and at most 60 percent by weight of the pigment blend. 20.The method of claim 14, wherein the pigment blend has a sediment voidvolume of at least 45 percent, wherein the sediment void volume ismeasured by preparing a pigment blend sample having the desired weightpercentage of the clay component to the calcium carbonate component;diluting the pigment blend sample with water to 50 percent by weightsolids to provide a slurry; placing a 70 gram sample of the slurry intoa centrifuge tube; spinning the sample at about 8000 g for about 90minutes; removing the sample from the centrifuge; separating the clearsupernatant liquid from the sample; weighing the clear supernatantliquid; determining the weight of water still contained in the voids ofthe sediment based on the weight of the clear supernatant liquidremoved; and using particle densities, converting the weight of water inthe voids into percent sediment void volume.